1. Field of the Invention
The present disclosure relates generally to a system and method for a synchronous rectifier drive, and relates more particularly to a low-cost synchronous rectifier drive for use in power converters, particularly flyback power converters operating in transition and discontinuous mode.
2. Description of Related Art
Synchronous rectifiers are typically implemented as a replacement for a diode operating alone to emulate diode operation with improvements in efficiency. Typically, a transistor switch with a low forward conducting resistance and high blocking impedance takes the place of a diode. The switch is turned off to block conduction in one direction, and switched on with current flowing in another direction to provide a low forward voltage drop for conducting current. The switching events are synchronized with voltage or current flows to emulate diode operation.
Applications for synchronous rectifiers include flyback DC-DC converters, such as a circuit 100 illustrated in FIG. 1. Circuit 100 in FIG. 1 operates with a diode D1 as a conventional rectifier on a secondary side of a transformer T1 having a number of turns Ns. Flyback converter 100 has an isolated topology with a power switch Q1 on a primary side of transformer T1 having a number of turns Np. Energy is stored in primary winding in power transformer T1 during the on-time of switch Q1, while a load current Iload is supplied by an output capacitor Cout. When switch Q1 is turned off, the energy stored in the primary winding is transferred to the secondary winding of transformer T1 and provided to the output as load current Iload. In addition, the secondary winding supplies a current Is that recharges capacitor Cout to replenish charge lost while capacitor Cout delivered output load current Iload.
In FIG. 2, a flyback converter 120 is illustrated as having a synchronous rectifier in the form of a MOSFET switch Q3 on a low side of the secondary side of transformer T1. Switch Q3 is coupled to a ground reference point to simplify the gate drive operation of switch Q3 for synchronous rectification. In the configuration of circuit 120, one technique for driving switches Q2 and Q3 is to provide complimentary signals to the switches, so that switch Q2 is conducting while switch Q3 is off, and vice versa. However, operating switches Q2 and Q3 in such a configuration presents difficulties for operation of the flyback converter in circuit 120.
One drawback with providing complementary signals to drive switches Q2 and Q3 is that current Is can become negative. If current Is can become negative, then under light or no load conditions the transformer output consists of a positive current pulse followed by an equivalent negative pulse, such that the average transformer output current is small or zero. In this circumstance, a substantial amount of energy is circulated between the input and output of flyback converter 120, resulting in energy losses.
By operating flyback converter 120 in discontinuous or transition mode, there is the potential to reduce the above-described energy losses. Operation in discontinuous or transition mode can be achieved by turning switch Q3 off near the point where current Is begins to reverse.
One technique to control the switching of switch Q3 to potentially reduce energy losses is to use a current transformer for sensing the current reversal on the secondary side of transformer T1. However, the use of a magnetic component to sense current adds complexity and cost to the circuit, so that such a sensing technique is not commercially attractive.
Other control techniques for switching switch Q3 to limit energy losses depend upon sensing a drain-to-source voltage Vds of switch Q3 to determine current flow direction. This technique usually provides for turning switch Q3 on if voltage Vds is positive, and turning switch Q3 off if voltage Vds is negative. Comparators are typically used to determine when voltage Vds is positive or negative to provide a switching control for switch Q3. However, an appropriate switching control for switch Q3 using this technique calls for the use of very high speed, high sensitivity comparators, that are difficult to implement and are prone to faulty operation in the presence of noise, such as typically exists in a flyback converter environment.
Accordingly, it is desirable to provide a relatively low cost synchronous rectifier controller for a flyback converter. In addition, it is desirable to provide a relatively low cost synchronous rectifier controller for other types of power converters, including forward converters. Numerous applications involving power conversion would benefit from such a controller.